The invention disclosed herein is generally related to a method of using a high frequency induction plasma tubes, and more specifically to a method for processing materials using an inductively coupled plasma. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
High frequency induction plasma tubes are well-known for producing high temperature gaseous plasmas. Such plasmas are useful in a number of practical applications including high temperature spectroscopic studies, preparation of microcrystalline refractory materials, and making powders.
In conventional methods of forming metal vapors using a plasma tube, metal, usually as a gaseous compound or powder, is introduced into a gaseous plasma. The plasma is formed from an ionizable gas such as argon. The metal vapor is expanded through a nozzle and deposited on a cold surface. In a similar manner, the metal vapor can be converted to a very fine powder.
This method of forming powders and coating surfaces has major drawbacks. A starting material cannot be introduced into conventional plasma tubes without onerous processing. Specifically, the starting material must be either liquefied or formed into an aerosol or gaseous compound.
A second disadvantage of conventional methods of forming powders is that a gas is necessary in order to form the plasma. The gas promotes voids when the plasma tube is used for metal coating surfaces.
The reason that gas is necessary is that induction plasma tubes known in the art generally include an electrical induction coil surrounding an enclosure which contains an ionizable gas. The coil is connected to a source of high frequency (400 kHz to 5 MHz) electrical power. A quartz tube centered inside the coil typically defines the enclosure. It is believed that this type of conventional "induction" plasma tube is actually capacitively coupled with the electrical induction coil and the gas being one of the capacitor plates and the quartz tube being the dielectric material. If metal is used to form the plasma in this type of plasma tube, the metal deposits on the quartz wall. The metal, in effect, becomes part of the dielectric material and prevents the plasma from "seeing" the alternating electric field produced by the coil. Almost immediately, the metal deposit burns through the quartz enclosure.
Upon application of power to the induction coil in conventional plasma tubes, the gas is ionized producing a central core of hot gaseous plasma inside the enclosure. At low power levels the plasma is concentrated in the center of the enclosure such that there is reduced danger of heat damage to the enclosure walls. At high power levels, however, the plasma core is both hotter and larger in diameter. As a result, the quartz enclosure is easily damaged by the plasma, which typically attains temperatures on the order of 10,000.degree. C. and above. This problem is aggravated by the fact that the plasma is typically subject to magnetic and electric instabilities that cause it to fluctuate in position and occasionally actually contact the enclosure walls. Operation at high power levels also results in the emission of intense ultraviolet radiation from the plasma, which ionizes the air around the enclosure and may result in electrical arcing in the induction coil.
These adverse effects have been overcome by the use of internal water-cooled shields as described and disclosed in U.S. Pat. No. 4,431,901 issued on Feb. 14, 1984, the teachings of which are hereby incorporated by reference. By using a thick segmented shield shaped in cross section to occlude line-of-sight transmissions of light, it is possible to get induction heating of the plasma because a current is induced around each of the individual segments. Without occluding line-of-sight transmissions of ultraviolet radiation from the plasma, the air around the windings is ionized which induces arcing of the plasma. A counterflow cooling system is used to cool the individual segments. Such an improved shielding system makes it possible to maintain a plasma at temperatures on the order of 10,000.degree. C.
U.S. Pat. No. 4,431,901 does not suggest the use of the plasma tube for coating materials or for making ultrafine ultrapure powders. In addition, this patent does not contemplate the use of a pure metal plasma and, in fact, only mentions the use of an ionizable gas to form the plasma.
Accordingly, it is an object of the present invention to provide a method for producing ultrafine, ultrapure powders in a gas free environment.
It is also an object of the present invention to provide a method of coating material with a pure metal.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.